The present invention relates to a light beam deviation detecting device used in an image forming apparatus, and more particularly to a technology for detecting deviation of a scanning position for each of a plurality of light beams in the sub-scanning direction in an image forming apparatus wherein a plurality of light beams are caused to scan a recording medium parallel in the primary scanning direction simultaneously so that plural lines may be recorded simultaneously.
In an image forming apparatus wherein a laser beam (light beam) modulated based on image signals is caused to scan on a recording medium for recording of image information after being deflected by a polygon mirror or the like, it is known that a plurality of laser beams are used for recording plural lines simultaneously for accelerating image recording (See Japanese Patent Publication Open to Public Inspection No. 188713/1990 (hereinafter referred to as Japanese Patent O.P.I Publication) and others).
However, when a plurality of laser beams are caused to scan simultaneously as stated above, it has sometimes happened that a scanning position of each laser beam deviated and a line interval for image recording thereby varied, resulting in detracted fidelity of image forming.
It is therefore desired that there is provided a device capable of detecting easily an amount of deviation of an interval of each laser beam in the sub-scanning direction from an expected value, and thereby a laser beam interval can be adjusted automatically, and thus the laser beam interval can be maintained at the expected value despite a change in environmental conditions and aging deterioration.
Further, when an actual scanning position is deviated from the reference scanning position in the sub-scanning direction, fidelity of image forming is deteriorated even when beam intervals can be adjusted to the expected value based on the results of the aforementioned detection of beam intervals deviations. Therefore, a device capable of detecting scanning positions in the sub-scanning direction easily has been desired.
In the aforementioned detection of an amount of deviation of a beam interval in the sub-scanning direction, there have been problems of a requirement that an amount of deviation needs to be detected within an expected resolving power and thereby necessary image recording accuracy needs to be maintained through adjustment of a laser beam scanning position. Further, there has been a problem that the necessary and lowest resolving power needs to be secured because of a problem that establishment of excessive accuracy for detecting an amount of deviation causes a cost increase of a sensor.